1. Field of the Invention
This invention relates to an automatic transmission system for automotive vehicle and, more particularly, to an automatic transmission system wherein four solenoid valves are adopted to control shift valves such that a construction of the system can be simplified and shift feeling can be improved.
2. Description of the Conventional Art
A conventional automatic transmission for an automotive vehicle has a torque converter and a multiple stage transmission gear mechanism connected to the torque converter. The torque converter includes hydraulically activated friction elements for selecting one of the gear stages of a transmission gear mechanism according to the vehicles operating condition.
A hydraulic control system pressurized by a hydraulic pump provides the hydraulic pressure required to operate the friction elements and control valves.
A commonly used automatic transmission of a vehicle has a hydraulic torque converter which generally includes a pump impeller connected with an engine output shaft to be driven thereby, a turbine runner with an output shaft member, and a stator disposed between the pump impeller and the turbine runner, so that the fluid is circulated by the engine driven pump impeller through the turbine runner with the aid of the stator which functions to deflect the fluid from the turbine runner to a direction where the fluid flow does not disturb the rotation of the pump impeller when the fluid flows into the pump impeller.
Automatic shifting is performed by operating the friction elements such as clutches or a kick-down brake at each shifting stage.
Also, a manual valve (where ports are converted by selecting a position of the selector lever), is supplied with fluid from the hydraulic pump and supplies the fluid to a shift control valve.
U.S. Pat. No. 3,754,482 discloses one of the automatic transmissions described above having first and second shifting devices, wherein two friction elements among four friction elements are activated, selectively, to perform several speed stages.
Because known automatic transmission systems of the kind explained above can perform four forward speed stages and one reverse speed stage only, it is difficult to improve power transmission efficiency; and, because the systems cannot perform skip shifting, they exhibit slow responsiveness.
These conventional automatic transmission systems have a complicated valve body construction. Thus, the valve body is expensive and difficult to manufacture because there are many valves in the systems. Also, shift feeling is not good when shifting a speed stage.